^ Adding to Martin's great post, D&D introduces further wrinkles:

"constructed some sort of fortification" - the beauty of magic is that most parties can easily do this without needing to prepare the battlefield in advance the way those medieval archers would have had to. Even very rudimentary battlefield control like grease, entangle and web count as "fortifications" to protect a party's ranged - which means they need a frontline of infantry a lot less. Buffs like Levitate or Mirror Image also count as "fortifications" in this context. The options grow exponentially as the characters level up, and that's when they don't simply conjure "infantry" (summons/pets) out of thin air.

"heavily specialized builds" - you really don't see these often in D&D since there's very little reward for doing so. You'd have to really go out of your way to be a "fire mage" or "healer" who can do nothing at all else - and why would you? Even if your backstory or something says you have an affinity for fire, in game terms that doesn't make non-fire spells any harder for you to learn, nor ineffective. You might get a feature of some kind that says you can do more with a specific kind of spell, but very few of those force you to be weaker with the others. Similarly, even the healbot-iest cleric to ever healbot knows his entire spell list and can whip out a spiritual weapon etc with at most a day's notice. And their power is even higher in previous editions.


In short - this paradigm doesn't work in D&D for a lot of reasons, and trying to reintroduce it would likely entail changing too much about the classes and system for it to be worthwhile.